Subsea wellheads normally have a large diameter conductor casing, such as a thirty inch casing, on which a housing is supported which housing extends upwardly and downwardly from the wellhead and is supported from the conductor casing. In low pressure drilling structures or high pressure production risers, external cyclic loads are imparted to the wellhead housing. It is preferred to transmit these cyclic loads from the wellhead housing to the conductor casing. When such cyclic loads are allowed to enter the smaller casing connected to the wellhead housing, fatigue in the smaller casing may result.
In prior structures large external bending loads are usually reacted through a force couple system created when the wellhead housing and a smaller casing (20") move relative to the conductor casing until contact is made. The force couple system is sometimes accomplished using bosses located on the wellhead housing and heavy wall extension that contact the conductor housing (large casing 30") when relative movement occurs. These bosses, however, require radical clearance to enable the wellhead housing to be run inside the conductor housing. The radial clearance between components allows the smaller casing to react those cyclic loads insufficient to move the wellhead housing enough to create the force couple. Reacting these loads in the smaller casing could result in fatigue of the smaller casing.
Also prior axial attachment of the wellhead housing and conductor casing is accomplished by one component carrying spring loaded latching segments, pins or a split ring that engage in a mating groove on the other component (e.g., U.S. Pat. Nos. 3,468,558 and 3,871,449). These attachment mechanisms, due to design clearances, allow axial movement of the wellhead housing. Axial movement of the wellhead housing could result in some load transfer mechanism, such as moment bosses or double angle tapers, to be disengaged when the smaller casing is cemented. Having the load transfer mechanism between the wellhead housing and conductor casing disengaged could create a fatigue problem in the smaller casing.